In paper board coating the operating speeds are increasing since productivity demands are getting constantly higher. This concerns also coating of paper. Very often a good coverage of the coated layer is of great importance for the quality of the final product. The coverage effects many quality properties of the final product such as gloss uniformity, uniform ink absoption, good and uniform brightness etc. The coverage is of extreme importance in coating of webs having low brightness such as cardboard or paper grades that have not been bleached in full and some grades where recycled fibers and low intensity bleaching are used.
An excellent device for producing such coatings is an air knife coater. In this apparatus the coating mix is spread on a moving web with an applicator roll or a nozzle applicator and the excess coating is scraped away with a thin air jet blown from an air knife. These apparatuses are well known in paper and board manufacturing. At present, the air knife is a bottleneck of the coating process due to its limited capacity to operate at the high speed required in modern processes. The air knife requires regular cleaning which leads to down time, the coating color solids are limited to 40-50% and the smoothness of the final product is poor according to present standards. The air knife also takes a lot of space in the lay-out, it is noisy and a lot of coating mix mist is entrained in the exhaust air causing cleaning problems. A typical problem relating to the quality of the final product in air knife coating is poor smoothness of the coating surface caused by the contour type coverage.
In air knife coating, there is a determinable dwell time between the application of the coating mix and the air knife doctoring. During this dwell time, a settled coating layer is formed on the web to be coated and excess free coating is blown away from the web by the air knife. The coating layer left on the web is contoured according to the form of the web surface and the coating layer thus has a very uniform thickness. Also the coat weights are normally high (8-22 g/m.sup.2). The blow off capacity of the air doctor limits the operation range of the coating apparatus to typical solids content of mineral coating mixes of 25-55% and to web speeds of 50-600 m/min.
A contoured type coverage can be achieved also with a film transfer coating apparatus. In such an apparatus the coating film is first made on a rotating roll and then transferred onto the surface of a web travelling on the film transfer roll. Film transfer coaters can operate at speeds up to 2000 m/min. and with solids contents up to 70%. However, film transfer apparatuses are not capable of producing high coat weights. A typical upper range for coat weight is 10-12 g/m.sup.2. The absorption characteristics of the applicator roll and the web determine the filmslit surface after the application nip and this determines the achievable maximum coat weight left on the web after the film split. In addition, the uniformity of the on the web is determined by the absorption characteristics of the web. If the absorption characteristics of the web change, for example due to moisture variation in the machine or cross-machine direction of the web, the quality of the final product may be effected.
The air knife coaters and film transfer coaters are not capable of producing as smooth coating as blade coaters which are superior in this aspect.